Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses in eye glasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant, transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated. Acrylics, such as polymethylmethacrylate, are also widely used transparent plastics for glazing.
Although these plastics provide the advantages of being more shatter-resistant and lighter than glass, they are more easily marred by contact with abrasives such as dust, cleaning equipment and ordinary weathering. This has led to the development of coatings which serve to improve the abrasion resistance of transparent plastics. For example, mar-resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225 (Misch, et al.); 3,976,497 (Clark), 3,986,997 (Clark); 4,027,073 (Clark); 4,159,206 (Armbruster, et al.) and 4,177,315 (Ubersax), for example, describe such compositions. Improved such compositions are also described in commonly assigned copending U.S. application Ser. No. 964,910, filed Nov. 30, 1978, now abandoned, and Frye, U.S. Pat. No. 4,277,287.
It has been discovered that such polysilicic acid coatings, especially if acidic, fail to adhere to certain plastic substrates, such as polycarbonate; and, even if prepared on the basic side of neutrality, they may adhere initially but peel after brief light aging. In copending application Ser. No. 91,716, now U.S. Pat. No. 4,299,746, the addition of an ultraviolet light absorbing compound, such as 2,4-dihydroxybenzophenone, is suggested, but in some cases this may have a plasticizing effect and polycarbonate seems to have a tendency to reject the coating on severe exposure. Another approach is to use a two-step system in which, for exmple, an acrylic primer is first applied which adheres to both the silicone resin coating and the polycarbonate substrate, and also serves as a binder for high levels of UV screens. The silicone resin coat is put on over the primer coat. See, for example, U.S. Pat. No. 4,188,451 (Humphrey, Jr.).
To avoid the need for a primer coat, commonly assigned copending U.S. application Ser. No. 34,164, filed Apr. 27, 1979, now U.S. Pat. No. 4,413,088 suggests replacing the usual alcohol solvents, e.g., isopropanol, with more aggressive solvents such as ketones, esters, nitroparrafins or the like. These solvents, however, are expensive and generally might require process modifications. Another approach to the elimination of primers is found in commonly assigned copending U.S. application Ser. No. 154,624, filed May 30, 1980, which discloses that incorporation of a silylated ultraviolet light screening compound into a silicone resin admits to primerless adhesion to plastics.
The above-mentioned patents and applications are incorporated herein by reference.